As one of conventional tire pressure detection apparatuses, a direct type is known. A tire pressure detection apparatus of this type includes transmitters each provided with a sensor such as a pressure sensor and directly attached to the wheel side, where tires are mounted. An antenna and a receiver are provided to the vehicle body side. When detection signals from the sensors are wirelessly transmitted from the transmitters, the detection signals are received by the receiver via the antenna, based on which the tire pressure is monitored.
In such a tire pressure detection apparatus, when the vehicle is detected to be running, it is desired that information regarding tire pressure be wirelessly transmitted from the transmitters to the receiver, irrespective of whether there is an abnormality in the tire pressure. Detection of a vehicle running state is achieved based on detection signals from acceleration sensors installed in the transmitters. When an acceleration based on which the vehicle is assumed to be running is detected, the transmitters wirelessly transmit the information regarding tire pressure.
However, with a conventional tire pressure detection apparatus, a running state of the vehicle cannot be detected unless the vehicle speed reaches a certain level (for example, 30 km/h or more) due to limitations such as large measurement errors in the acceleration sensors equipped in the transmitters, because of which detection of a vehicle speed is difficult at low speed.
Patent Literature 1 proposes a tire pressure detection apparatus capable of detecting a running state of the vehicle at lower speed. In this tire pressure detection apparatus, a variance value of measured values of acceleration when the vehicle is at a standstill, i.e., when the tire is not rotating, is used as a value of the standstill state, and the vehicle is determined to be running when this variance value is increased. With this configuration, a running state can be detected at lower speed, and the information regarding tire pressure can be wirelessly transmitted from the transmitters at lower speed.
However, since the transmitters are attached to the wheels in direct type tire pressure detection apparatuses, and powered by small batteries provided inside the transmitters, an increase in power consumption needs to be minimized. In the tire pressure detection apparatus described in Patent Literature 1, in particular, the number of measurements is increased in order to detect the running state of the vehicle at low speed, so that the power consumption is increased.